In the high voltage research and application field, it is often concerned of very fast transient high voltage, in which the voltage amplitude can be up to several million volts, the voltage rising time is within a range of several nanoseconds and the frequency can be up to decades of MHz. It makes an important and practical sense to exactly measure very fast transient high voltage.
For example, in electric power system, Gas Insulation Switchgears (GIS) are used in large quantities, the operation of GIS is concerned the problem of very fast transient high voltage. It is described in the following with reference to FIG. 1: three sections of central conductors 11a, 11b and 11c of GIS busbar are supported with the insulators 13 within the metal housing 12, and the movable contacts 14a and 14b of the switches can protrude into and retract from the central conductors 11a and 11c, so as to complete the closing or opening operation between the central conductors. When the central conductor 11a is connected with a power-frequency power supply, the movable contact 14b is in an open state and the movable contact 14a is in a operating process, there might appears such a phenomena in which there appears a high voltage at the gap between the central conductor 11a and 11b so that the gap is broken down and very fast transient high voltage is produced at the busbar, the amplitude of which might far and away exceed the rated voltage of the equipment, so it is called very fast transient over-voltage (VFTO). VFTO severely endangers the insulations of the substation equipment. It is necessary to exactly measure the waveform of VFTO when researching and solving the problem of VFTO.
Conventional methods for measuring VFTO use resistance voltage divider or capacitive voltage divider etc. In the following is explained a method for measuring VFTO of the capacitive voltage divider in GIS. As shown in FIG. 1, when measuring VFTO at position of measurement point 15 on busbar, a measurement system consisted of a capacitive voltage divider 16 and a measurement device 17 is used. Said capacitive voltage divider 16 has a structure as shown in FIG. 2, a hand hole 21 is made in the housing 12 of the GIS corresponding to the position of measurement point, an inductive electrode 22 is fixed in the hand hole 21, the inductive electrode 22 and the GIS housing 12 are insulated from each other, and the stray capacitance between the inductive electrode 22 and the high voltage busbar 11b forms the high voltage arm of the capacitive voltage divider 16, the stray capacitance between the inductive electrode 22 and the GIS housing 12 forms the low voltage arm of the capacitive voltage divider 16, and voltage of the low voltage arm is inputted into the measurement device 17 via cable 24.
The capacitive voltage divider 16 converts the very fast transient high voltage to a low voltage signal satisfying the requirement of the measurement device 17 such as an oscilloscope. In order to ensure exact measurement of very fast transient high voltage, it is required to calibrate exactly the frequency response characteristic and the voltage dividing ratio of the measurement system. A typical parameter for describing the frequency response characteristic of the measurement system is “rising time of step response”, that is, the time in which the system response changes from the lower voltage level to the higher voltage level when the system is excited under a step voltage. The rising time of step response describes the response speed and measurement capability of the measurement system to a fast transient voltage; the voltage dividing ratio is the ratio of the practical very fast transient high voltage to the converted low voltage; and the calibration means assigning a certain parameter value to the calibrated measurement system, herein assigning the value of the rising time of step response and the voltage dividing ratio to the measurement system.
The measurement system consists of the capacitive voltage divider 16 and the measurement device 17, its frequency response characteristic and voltage dividing ratio can be calibrated via experiments, the process is: applying a very fast transient high voltage of a known waveform at a measurement point of the busbar, comparing the known waveform and the waveform measured by the calibrated measurement system; and determining the frequency response characteristic and the voltage dividing ratio of the calibrated measurement system. The very fast transient high voltage of known waveform is the calibration voltage. For the exact calibration, the calibration voltage requires a waveform not only having enough high rising gradient of wave front, but also having enough high voltage amplitude, so that the output of the measurement system has enough high signal amplitude and signal-to-noise ratio.
Prior art provides a calibration method: providing a high voltage steep pulse generator which outputs a calibration voltage with the voltage rising time in the range of several ns; measuring the calibration voltage simultaneously with an existing measurement system satisfying the measurement accuracy requirement and the calibrated measurement system; making comparison between the measurement results of the two measurement systems and determining the frequency response characteristic and voltage dividing ratio the calibrated measurement system.
Prior art also provides a calibration method: providing a high voltage steep pulse generator which outputs a calibration voltage with known waveform and amplitude, of which the voltage rising time is in the range of several ns; the high voltage steep pulse generator is connected to the measurement point via a transition connector, and outputs impulses to act upon the calibrated measurement system; in order to keep the voltage waveform of the measurement point the same as the voltage waveform of the calibration source, the reflection of impulses in the transition connector must be avoided, therefore, the transition connector must be well transited and matched in wave impedance.
In the above calibration methods in the prior art, it is necessary to use a high voltage steep pulse generator, and to measure the calibration voltage using an existing measurement system satisfying the measurement accuracy requirement, and also to complicatedly match in wave impedance, so the system is of complicated structure and the measurement process is very cumbersome.